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Plant Pigment Puzzle Complete

Once the biochemistry of flower color is known for a specific plant, it
will be possible to create an infinite range of custom-colored flowers using
genetic engineering, says ARS plant geneticist Robert Griesbach.
(K1897-1)

For about a century, scientists have tried to put together the many pieces
of the complicated puzzle of what gives flowers color.

Now, Agricultural Research Service scientists working with color genes have
placed the final piece in the puzzle.

"Genetic engineering can be used to create novel flower colors. So
foreign flower colors can now be easily introduced into most species,"
says flower pigment expert Robert J. Griesbach.

For the last 15 years, this ARS plant geneticist has been working on the
plant color puzzle in the U.S. National Arboretum's Floral and Nursery Plants
Research Unit at Beltsville, Maryland.

Says Griesbach, "To create a new palette of flower colors, a thorough
understanding of the biochemistry and genetics of flower color was
necessary."

Using petunias, he showed that specific shades of flower color could be
explained by the combined inheritance of plant pigments called flavonoids and
cell acidity, or pH.

Three different pigmentschlorophyll, flavonoids, and
carotenoidsmixed in different proportions, give color to flowers.

"By mixing and matching the three pigments, an endless array of
different colors can be created," Griesbach says. For example, most red
phalaenopsis (orchids) are the result of mixing orange carotenoids with magenta
flavonoids.

Chlorophyll, responsible for green color, is located in small packets called
chloroplasts. The carotenoids that impart yellow through orange colors are
found within other small cell packets called chromoplasts.

Unlike those two pigments, flavonoids are located within the vacuole, or
water-storage area at the center of a plant cell. They make up three-quarters
of each flower cell and are responsible for red through blue colors. Flavonoids
are divided into two groupspigmented anthocyanins and colorless
co-pigments.

While very little is known about the biochemistry of chlorophyll and
carotenoids as related to flower color, says Griesbach, we have a
lot of information about flavonoid chemistry and flower color. Flavonoid
research is 20 years ahead of carotenoid research.

Once the biochemistry of flower color is known for a specific plant,
it will be possible to create an infinite range of custom-colored flowers using
genetic engineering, he says.

"The drawback is the cost of doing the chemical analysis necessary to
create novel plants like blue roses."

Plant flower color is also influenced by the cellular
environmentespecially the pH of the plant vacuole and the presence of
metal ions, says Griesbach.

"In flowers, flavonoids are arranged like a sandwich. Picture the
anthocyanins as the filling and the co-pigment as the bread. Holding this
sandwich together, like mayonnaise, are metal ions that keep the structure from
falling apart.

"Cellular pH determines the space between the sandwichsquashing
or widening the stacking," he says. "Changing the distance between
the 'bread' changes the color."

For example, changing the pH in petunias by just one-tenth changes their
color from blue to red.

Griesbach explains that his color-triggering discovery built on the
pioneering biochemical research in the 1960's by a group of ARS scientists at
Beltsville headed by Sam Asen. Their experiments showed that roses and
cornflowers both have some of the same anthocyanin. But one is red, and the
other is blue.

The earlier researchers demonstrated how subtle changes in cell pH during
the day were responsible for the flower's changing colorspink as a bud,
blue in open bloom, and pink again when the bloom wilted. Their discovery led
to the identification of co-pigment and the role of pH in flower colors.

More recently, the physics of the role of pH in plant color was explained by
two Japanese scientists, T. Kondo and T. Goto. They used morning-glories to
show that cell pH affects color.

Griesbach says that the same principle causes color changes in hydrangeas.
"A soil pH of 6.0 will produce pink flowers, while a pH of 5.5 will
produce blue ones," he says. "The soil pH does not influence the pH
of the flower cells. But when soil is acidic (around 5 pH), aluminum becomes
more soluble and is taken up by the plant. When aluminum binds to the
anthocyanin/co-pigment complex, it changes the hydrangea's color from pink to
blue.

"Light and temperature can also affect flower color. Bright light and
cool temperatures during flower development can make blooms more vibrant."
-- By Hank Becker, ARS.

Robert J.
Griesbach is in the USDA-ARS Floral and Nursery Plants Unit, U.S. National
Arboretum, Beltsville, MD 20705-2350; phone (301) 504-6574.

"Plant Pigment Puzzle Complete" was published in the
July 1996
issue of Agricultural Research magazine.